1. Field of the Invention
The present invention relates to a distributed optical amplifier and a distributed optical amplifying method for adjusting an incident power of the light to be amplified depending on the optical power of pump light, considering the non-linear optical effects generated in the light to be amplified.
Moreover, the present invention relates to an optical communication system that can improve the transmission characteristic by utilizing such distributed optical amplifier.
An ultra-long distance and large capacity optical communication apparatus is now requested, aiming at establishment of future multimedia network. Development and research are continued for wavelength-division multiplexing (hereinafter abbreviated as “WDM”) as the system to realize large capacity transmission system from the advantageous viewpoint of wide frequency band of optical fiber and effective use of large capacity.
Particularly, in the ultra-long distance optical communication system, since the WDM optical signal is attenuated during transmission through the optical transmission line, the WDM signal must be amplified.
2. Description of the Related Art
The optical communication system of the related art comprises a transmitting terminal for generating the WDM optical signal formed by wavelength-division multiplexing of a plurality of optical signals of different wavelengths, an optical transmission line for transmitting the WDM optical signal transmitted from the transmitting terminal and receiving terminal and a receiving terminal for receiving the transmitted WDM optical signal and moreover this optical communication system also comprises, as required, one or a plurality of repeaters having the function to amplify the WDM optical signal in the course of the optical transmission line.
In such optical communication system, waveform of each optical signal is deteriorated due to the non-linear optical effects in the optical transmission line. In order to eliminate deterioration of waveform, it is effective to reduce the optical power (optical intensity) of the WDM optical signal incident to the optical transmission line, but reduction of optical power results in deterioration of signal to noise ratio (hereinafter, referred to as “optical SNR”). As the non-linear optical effects, for example, self phase modulation (hereinafter, referred to as “SPM”), cross-phase modulation (hereinafter, referred to as “XPM”), four-wave mixing (hereinafter, referred to as “FWM”), stimulated Raman scattering (hereinafter, referred to as “SRS”) and stimulated Brillouin scattering (hereinafter, referred to as “SBS”) are known.
For this purpose, it has been proposed to use in combination a centralized optical amplifier provided within a repeater and a distributed optical amplifier using the optical transmission line in common as the optical amplifying medium. For example, effectiveness of the Raman amplification is reported in P. B. Hansen, A. Stentz, T. N. Nielsen, R. Espinodola, L. E. Nelson, A. A. Abramov, “Dense wavelength-division multiplexed transmission in “zero-dispersion” DSF by means of hybrid Raman/erbium-doped fiber amplifier” (OFC/100C. '99), PD8, 19999 and N. Takachio, H. Suzuki, H. Masuda and M. Koga “32*10 Gb/s distributed Raman amplification transmission with 50-GHz channel spacing in the zero-dispersion region over 640 km of 1.55-μm, dispersion-shifted fiber” (OFC/100C '99), PD9, 1999.
Moreover, the Japanese Published Unexamined Patent Application No. HEI 03-013836 (Japanese Patent Application No. HEI 01-149148) discloses a method of Raman amplification by obtaining loss of the optical transmission line through detection of rear scattering of the incident test light to the optical transmission line.
The Japanese Published Unexamined Patent Application No. HEI 10-073852 (Japanese Patent Application No. HEI 08-232376) discloses the Raman amplification in the widened amplification band using a plurality of pump lights in different wavelengths.
The Japanese Published Unexamined Patent Application No. HEI 10-073852 (Japanese Patent Application No. HEI 08-170183) discloses inclusion of a pump light source for Raman amplification within a repeater.
Here, an optical amplifier may be classified into a centralized optical amplifier and a distributed optical amplifier. The centralized optical amplifier is an optical amplifier wherein an optical amplifying medium and a pump light source are centralized in one area. For example, a semiconductor laser amplifier and an optical fiber amplifier that is formed by winding an optical fiber as an amplifying medium around a bobbin are well known. On the other hand, the distributed optical amplifier is an optical amplifier wherein an optical amplifying medium is laid for a constant distance and the pump light source is provided in one or both areas. For example, an optical fiber amplifier is proposed. As an optical fiber amplifier, rare-earth element added optical fiber amplifier and an optical fiber amplifier utilizing the non-linear scattering in the optical fiber are proposed.
These centralized optical amplifier and distributed optical amplifier are identical in the physical process to amplify the optical signal but are mainly different in such a point that the optical amplifying medium is summarized within one area or is distributed for a constant distance. The distributed optical amplifier is characterized in that the optical amplifying medium can also be used as the inter-terminal optical transmission line for transmitting the optical signal.
Moreover, as the non-linear scattering, SRS and SBS are known. SRS is the scattering generated due to the mutual effect of optical phonon of lattice vibration and has wide gain width and large frequency shift. While, SBS is the scattering generated due to the mutual effect of acoustic phonon of lattice vibration and has gain width narrower than that of SRS and small frequency shift but has the gain efficient larger than that by two digits or more.
The optical fiber amplifier using the non-linear scattering is characterized in that an ordinary optical fiber such as NZ-DSF and SMF can be used, the pumping wavelength can be set for any amplification wavelength and gain is matched in the polarizing direction of the pump light. As the ordinary optical fibers, for example, distributed shift fiber (hereinafter, abbreviated as “DSF”), non-zero distributed shift optical fiber (hereinafter, abbreviated as “NZ-DSF”), distributed flat optical fiber (hereinafter, abbreviated as “DFF”) and 1.3 μm zero-distributed (normal distribution) single mode optical fiber (hereinafter, abbreviated as “SMF”) are proposed.
The references cited above disclose the Raman amplification but does not disclose the practical method how to control the optical power of the pump light and the optical power of light to be amplified with the distributed optical amplifier. Otherwise, if such reference discloses the practical method for such control, such reference requires a complicated circuit for the control of optical power.